In recent years, a light-emitting element using an organic compound as a light-emitting substance (the light-emitting element is also referred to as an organic EL element) has been actively researched and developed. In a basic structure of the light-emitting element, a layer containing a light-emitting substance is provided between a pair of electrodes. Voltage application to this element causes the light-emitting substance to emit light.
The light-emitting element is a self-luminous element and thus has advantages over a liquid crystal display element, such as high visibility of the pixels and no need of backlight, and is considered to be suitable as a flat panel display element. Another major advantage of the light-emitting element is that it can be fabricated to be thin and lightweight. Besides, the light-emitting element has an advantage of quite high response speed.
Since the light-emitting element can be formed in a film form, planar light emission can be provided; thus, a large-area element can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source applicable to a lighting device and the like.
In the case of a light-emitting element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, by applying a voltage to the element, electrons from a cathode and holes from an anode are injected into the layer containing the organic compound and thus a current flows. The injected electrons and holes then lead the organic compound to its excited state, so that light emission is provided from the excited organic compound.
The excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state (S*) is called fluorescence, and light emission from the triplet excited state (T*) is called phosphorescence. The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3.
At room temperature, a compound capable of converting a singlet excited state into light emission (hereinafter, referred to as a fluorescent compound) exhibits only light emission from the singlet excited state (fluorescence), and light emission from the triplet excited state (phosphorescence) cannot be observed. Accordingly, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
In contrast, a compound capable of converting a triplet excited state into light emission (hereinafter, referred to as a phosphorescent compound) exhibits light emission from the triplet excited state (phosphorescence). Furthermore, since intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can be theoretically increased to 100%. That is, higher emission efficiency can be achieved than using a fluorescent compound. For this reason, light-emitting elements using a phosphorescent compound have been under active development recently so that high-efficiency light-emitting elements can be achieved.
When a light-emitting layer of a light-emitting element is formed using the phosphorescent compound described above, in order to inhibit concentration quenching or quenching due to triplet-triplet annihilation of the phosphorescent compound, the light-emitting layer is usually formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound serving as the matrix is called host material, and the compound dispersed in the matrix like the phosphorescent compound is called guest material.
When a phosphorescent compound is a guest material, a host material needs to have higher triplet excitation energy (energy difference between a ground state and a triplet excited state) than the phosphorescent compound.
Furthermore, since singlet excitation energy (energy difference between a ground state and a singlet excited state) is higher than triplet excitation energy, a substance that has high triplet excitation energy also has high singlet excitation energy. Thus, the above substance that has high triplet excitation energy is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance.
Studies have been conducted on compounds having dibenzo[f,h]quinoxaline rings, which are examples of the host material used when a phosphorescent compound is a guest material (e.g., see Patent Documents 1 and 2).